1. Field of the Invention
The invention concerns an illumination system according to the preamble of claim I as well as a projection exposure unit with such an illumination system.
In order to be able to further reduce the structural widths of electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. Lithography with soft x-ray radiation, so-called EUV (extreme UV) lithography, is conceivable at wavelengths smaller than 193 nm, for example.
2. Description of the Prior Art
An illumination system suitable for EUV lithography will illuminate with as few reflections as possible the field provided for EUV lithography, particularly the annular field of an objective in a homogeneous manner, i.e., uniformly; further, the aperture diaphragm or pupil of the objective will be illuminated independent of field up to a specific filling ratio .sigma. and the exit pupil of the illumination system will lie in the entrance pupil of the objective.
An illumination system for a lithographic device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least 4 pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided, and the point plasma light source is imaged in an annular illuminated surface by means of a condenser, which has five aspherical mirrors arranged off-center. The annularly illuminated surface is then imaged in the entrance pupil by means of a special subordinate sequence of grazing-incidence mirrors.
From U.S. Pat. No. 5,581,605, an illumination system has been made known, in which a photon beam is split into a multiple number of secondary light sources by means of a honeycomb condenser. In this way, a homogeneous or uniform illumination is achieved in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of a conventional reduction optics. A gridded mirror is precisely provided with equally curved elements in the illumination beam path. The contents of the above-mentioned patents are incorporated by reference.